Project Summary: Impaired working memory is a core cognitive deficit of schizophrenia and other mental disorders and is strongly linked to patient outcome. Currently, there is no available treatment for deficits in working memory. Working memory, as studied here, is an observable behavior meeting the criteria of the Research Domain Criteria (RDoC) Initiative of the NIMH. The immediate goal of this application is to identify the neuronal circuitry and mechanisms that supports spatial working memory in the mouse. Identifying of these mechanisms will not only be important for understanding working memory but also for understanding basic principles of prefrontal circuit function. The long-term goal of this application is to guide the development of more effective therapeutic strategies for deficits in working memory and other mental processes that are thought to arise from prefrontal cortex dysfunction. Our published data from optogenetic and in vivo physiology studies link thalamo-prefrontal circuit function to working memory. Specifically, they indicate an important role for the medio-dorsal thalamus in sustaining elevated activity in the prefrontal cortex for working memory maintenance. However, the exact circuit architecture of this interaction is unclear, especially with regard to which cortical layers and cortico-thalamic projections are involved. Here, we would like to determine whether reciprocal connections between deep layers and the medio- dorsal thalamus are sufficient to support working memory (hypothesis 1) or whether the maintenance circuit involves an open-loop configuration including superficial layers (hypothesis 2). Moreover, we propose to test whether layer 6 corticothalamic neurons support working memory maintenance (hypothesis 3) whereas layer 5 corticothalamic neurons are important for retrieval/choice selection (hypothesis 4). We will combine layer specific Cre mice with inhibitory opsins and in vivo Ca2+ imaging to address the following two aims: Aim 1: To determine the significance of deep versus superficial cortical layers in spatial working memory Aim 2: To characterize the representation of working memory in different cortical layers This R21 application will set the basis for studying the circuit architecture regulating working memory in the mouse. Abnormalities in thalamo-prefrontal circuitry have been observed in several psychiatric disorders and are thought to be causally involved in the generation of cognitive deficits. Studying how this circuitry regulates cognition will therefore be essential for understanding cognitive deficits.